3,645 research outputs found
Surface modification of hydrophobic polymers for improvement of endothelial cell-surface interactions
The aim of this study is to improve the interaction of endothelial cells with polymers used in vascular prostheses. Polytetrafluoroethylene (PTFE; Teflon) films were treated by means of nitrogen and oxygen plasmas. Depending on the plasma exposure time, modified PTFE surfaces showed water-contact angles of 15¿58° versus 96° for unmodified PTFE. Electron spectroscopy in chemical analysis (ESCA) measurements revealed incorporation of both nitrogenand oxygen-containing groups into the PTFE surfaces, dependent on the plasma composition and exposure time. In-vitro biological evaluation of unmodified and modified PTFE surfaces showed that human endothelial cells, seeded from 20% human serum-containing culture medium, adhered well on to modified PTFE surfaces, but not on to unmodified films. Adhesion of endothelial cells on to expanded PTFE graft material (Gore-Tex) was also stimulated by plasma treatment of this substrate. On plasma-treated expanded PTFE, the adhering endothelial cells formed a monolayer, which covered the textured surface. The latter observation is important in view of the hemocompatibility of vascular grafts seeded with endothelial cells before implantation
Magnetic vortex oscillator driven by dc spin-polarized current
Transfer of angular momentum from a spin-polarized current to a ferromagnet
provides an efficient means to control the dynamics of nanomagnets. A peculiar
consequence of this spin-torque, the ability to induce persistent oscillations
of a nanomagnet by applying a dc current, has previously been reported only for
spatially uniform nanomagnets. Here we demonstrate that a quintessentially
nonuniform magnetic structure, a magnetic vortex, isolated within a nanoscale
spin valve structure, can be excited into persistent microwave-frequency
oscillations by a spin-polarized dc current. Comparison to micromagnetic
simulations leads to identification of the oscillations with a precession of
the vortex core. The oscillations, which can be obtained in essentially zero
magnetic field, exhibit linewidths that can be narrower than 300 kHz, making
these highly compact spin-torque vortex oscillator devices potential candidates
for microwave signal-processing applications, and a powerful new tool for
fundamental studies of vortex dynamics in magnetic nanostructures.Comment: 14 pages, 4 figure
Large microwave generation from d.c. driven magnetic vortex oscillators in magnetic tunnel junctions
Spin polarized current can excite the magnetization of a ferromagnet through
the transfer of spin angular momentum to the local spin system. This pure
spin-related transport phenomena leads to alluring possibilities for the
achievement of a nanometer scale, CMOS compatible and tunable microwave
generator operating at low bias for future wireless communications. Microwave
emission generated by the persitent motion of magnetic vortices induced by spin
transfer effect seems to be a unique manner to reach appropriate spectral
linewidth. However, in metallic systems, where such vortex oscillations have
been observed, the resulting microwave power is much too small. Here we present
experimental evidences of spin-transfer induced core vortex precessions in
MgO-based magnetic tunnel junctions with similar good spectral quality but an
emitted power at least one order of magnitude stronger. More importantly,
unlike to others spin transfer excitations, the thorough comparison between
experimental results and models provide a clear textbook illustration of the
mechanisms of vortex precessions induced by spin transfer
Microwave Oscillations of a Nanomagnet Driven by a Spin-Polarized Current
We describe direct electrical measurements of microwave-frequency dynamics in
individual nanomagnets that are driven by spin transfer from a DC
spin-polarized current. We map out the dynamical stability diagram as a
function of current and magnetic field, and we show that spin transfer can
produce several different types of magnetic excitations, including small-angle
precession, a more complicated large-angle motion, and a high-current state
that generates little microwave signal. The large-angle mode can produce a
significant emission of microwave energy, as large as 40 times the
Johnson-noise background.Comment: 12 pages, 3 figure
The optical afterglow of the short gamma-ray burst GRB 050709
It has long been known that there are two classes of gamma-ray bursts (GRBs),
mainly distinguished by their durations. The breakthrough in our understanding
of long-duration GRBs (those lasting more than ~2 s), which ultimately linked
them with energetic Type Ic supernovae, came from the discovery of their
long-lived X-ray and optical afterglows, when precise and rapid localizations
of the sources could finally be obtained. X-ray localizations have recently
become available for short (duration <2 s) GRBs, which have evaded optical
detection for more than 30 years. Here we report the first discovery of
transient optical emission (R-band magnitude ~23) associated with a short
burst; GRB 050709. The optical afterglow was localized with subarcsecond
accuracy, and lies in the outskirts of a blue dwarf galaxy. The optical and
X-ray afterglow properties 34 h after the GRB are reminiscent of the afterglows
of long GRBs, which are attributable to synchrotron emission from
ultrarelativistic ejecta. We did not, however, detect a supernova, as found in
most nearby long GRB afterglows, which suggests a different origin for the
short GRBs.Comment: 11 pages, 3 figures, press material at http://www.astro.ku.dk/dark
The effects of over-expression of the FK506-binding protein FKBP12.6 on K+ currents in adult rabbit ventricular myocytes
This study examines the effects of the intracellular protein FKBP12.6 on action potential and associated K+ currents in isolated adult rabbit ventricular cardiomyocytes. FKBP12.6 was over-expressed by ~6 times using a recombinant adenovirus coding for human FKBP12.6. This over-expression caused prolongation of action potential duration (APD) by ~30%. The amplitude of the transient outward current (Ito) was unchanged, but rate of inactivation at potentials positive to +40 mV was increased. FKBP12.6 over-expression decreased the amplitude of the inward rectifier current (IK1) by ~25% in the voltage range −70 to −30 mV, an effect prevented by FK506 or lowering intracellular [Ca2+] below 1 nM. Over-expression of an FKBP12.6 mutant, which cannot bind calcineurin, prolonged APD and affected Ito and IK1 in a similar manner to wild-type protein. These data suggest that FKBP12.6 can modulate APD via changes in IK1 independently of calcineurin binding, suggesting that FKBP12.6 may affect APD by direct interaction with IK1
Genetic noise control via protein oligomerization
Gene expression in a cell entails random reaction events occurring over
disparate time scales. Thus, molecular noise that often results in phenotypic
and population-dynamic consequences sets a fundamental limit to biochemical
signaling. While there have been numerous studies correlating the architecture
of cellular reaction networks with noise tolerance, only a limited effort has
been made to understand the dynamic role of protein-protein interactions. Here
we have developed a fully stochastic model for the positive feedback control of
a single gene, as well as a pair of genes (toggle switch), integrating
quantitative results from previous in vivo and in vitro studies. We find that
the overall noise-level is reduced and the frequency content of the noise is
dramatically shifted to the physiologically irrelevant high-frequency regime in
the presence of protein dimerization. This is independent of the choice of
monomer or dimer as transcription factor and persists throughout the multiple
model topologies considered. For the toggle switch, we additionally find that
the presence of a protein dimer, either homodimer or heterodimer, may
significantly reduce its random switching rate. Hence, the dimer promotes the
robust function of bistable switches by preventing the uninduced (induced)
state from randomly being induced (uninduced). The specific binding between
regulatory proteins provides a buffer that may prevent the propagation of
fluctuations in genetic activity. The capacity of the buffer is a non-monotonic
function of association-dissociation rates. Since the protein oligomerization
per se does not require extra protein components to be expressed, it provides a
basis for the rapid control of intrinsic or extrinsic noise
- …